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Re: Action-67 some examples on b-nodes issues and their impact on users

From: Jeff Z. Pan <jpan@csd.abdn.ac.uk>
Date: Fri, 25 Jan 2008 21:45:21 +0000
Message-ID: <479A5871.4080202@csd.abdn.ac.uk>
To: Ivan Herman <ivan@w3.org>
CC: public-owl-wg@w3.org

> thanks a lot. These types of down-to-Earth examples help me at least 
> (modulo Peter's comment that I still have to digest:-).
> In trying to grasp and understand the consequences of the 'existential 
> vs. skolem' proposal/discussion, I must admit that I found some gaps 
> in my own understanding. I have therefore some questions; if you, 
> Boris, Bijan, Jeremy, or the others could answer those it would help 
> me at least....
> 1. Scope of skolemization
> I am not sure I fully understand the proposal in terms of the 'skope' 
> of the skolemization. By that I mean: what are the 'units' (I do not 
> know how to call this) within which two 'identical' blank nodes are 
> skolemized with the same new URI? For OWL:

I am not sure if I get the question.

>  - are we speaking about an 'ontology' as being one 'unit'? Or are the 
> ABox and TBox separated in this sense? I heard different remarks used 
> on the calls, that is why I ask (I may have misunderstood something).

In general, axiom is the unit of ontology.  An ABox is a set of 
individual axioms, while a TBox is a set of class/property axioms. Given 
an ontology O and its ABox A and its TBox T, O is the union of A and T.
>  - how does this affect the import mechanism? Is skolemization done 
> after or before all imports? (I would expect 'after', but I just 
> wanted to be sure...)
After, so to speak.

>  - I expect that the 'left' side and the 'right' side of an inference 
> are skolemized separately (this is what one of Jeremy's test case 
> says), but I also heard remarks on the call that only the left side is 
> skolemized and the right side isn't... Or, again, did I misunderstand 
> something?

Could you point out which test case from Jeremy that you refer to?


P.S. I assume that the following questions are extensively discussed 

> 2. RDFS
> I would like to understand how exactly the proposed changes affect the 
> semantic compatibility of OWL-Full and RDFS, ie, where would the 
> differences be. My current understanding is that the semantics of OWL 
> Full is a suitable extension of the RDFS semantics, ie, everything 
> that is true in RDFS is also true in OWL Full. Will that relationship 
> break? I would expect so, but I would like to have examples on how 
> this would diverge... Jeff, would it be possible to find an example 
> where an RDFS inference would no longer be true in a changed OWL1.1 Full?
> A related question is whether it is possible to change the semantics 
> of all this on the DL interpretation only. There might be still 
> backward incompatibilities between OWL1.1 DL and OWL1.0 DL (I guess 
> your example is one of those) but it might restrict them so that it 
> would not affect, RDFS+OWL Full usage too much... Would that line make 
> sense at all?
> Thanks
> Ivan
> Jeff Z. Pan wrote:
>>> Action 67: Jeff to lead effort on formulating some examples on
>> b-nodes issues and their impact on users
>> As Boris pointed out in the telecon, there was already a nice
>> example (hidden behind some rather technical discussions) in his
>> earlier email:
>> http://lists.w3.org/Archives/Public/public-owl-wg/2007Nov/0177.html
>> In short, there are two choices for semantics of anonymous
>> individuals (b-nodes):
>> 1) existentially quantified variables
>> 2) skolem constants
>> Example:
>> Given an ontology O about friends (suppose there are no anonymous
>> individuals in O). Let us consider the following extra individual
>> axioms (where :_1 is an anonymous individual):
>> hasFriend(Bob,:_1)
>> hasAge(:_1,"26"^^xsd:integer)
>> With both semantics, the axioms both roughly say "Bob has some
>> friend aged 26" with some subtle difference: under semantics 1),
>> the friend aged 26 could be someone already mentioned in O, while
>> under semantics 2), the friend is someone new and cannot be
>> someone mentioned in O.
>> (The above is true unless we have some further extra axioms
>> forcing :_1 to be the same as some known individuals.)
>> Comments/Further examples are welcome.
>> Jeff
Received on Friday, 25 January 2008 21:46:05 UTC

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